1. Field of the Invention
The present invention relates to a method of monitoring the position of a sensor.
2. Description of the Related Art
Sensors for measuring the presence of one or several components in a fluid have been known for several decades. The components may be for example ions or in the form of gas and be present in various fluids. The sensors may be based on various measuring principles, for example electrochemical measuring principles such as potentiometry, amperometry and polarography, and optical measuring principles.
It is common to most of the sensors that at a time during use they must be located in a certain position, and it may be of crucial importance that the sensor is located correctly. To ensure this, many systems are provided with position detectors. The determination of the position may be to detect whether the sensor is located correctly in a measuring chamber, as for example in measuring devices wherein a replaceable sensor is located in a combined measuring and calibration chamber built into a monitor. The determination may also simply comprise detection of where in the system the sensor is, as for example in measuring devices for in vivo measurements where the sensor during measuring is in direct contact with a patient and during calibration is located in a separate calibration chamber. To ensure that the sensor is not calibrated when located on a patient, it may be necessary automatically to check whether the sensor is present in the calibration chamber when calibration is activated.
An example of such measuring device is the monitor of the type TCM3 from Radiometer A/S, Copenhagen, Denmark. Said monitor is adapted for transcutaneous (tc) measuring of the O.sub.2 and/or CO.sub.2 content in blood. Calibration of a sensor in the form of an electrode connected to the monitor is performed with the electrode located in a calibration chamber placed in the monitor itself to which the calibration gas is supplied through a tube assembly from a separate calibration device.
To ensure that calibration of the electrode is not performed while the electrode is located on a patient, a micro switch is built into the calibration chamber, detecting whether the electrode is located in the chamber. Thus, activation of the calibration is only possible when the micro switch detects that the electrode is in position in the chamber.
Apart from controlling the possibility of calibration, the information from the micro switch may be used to control several other functions in the monitor as well. With the electrode located in the calibration chamber, the heat supply to the electrode is shut off after 30 minutes in the chamber, ensuring that the electrolyte between the electrode and the membrane does not evaporate disproportionately quickly when the electrode is not used. Moreover, other functions--necessary only when measuring--are disconnected, for example the acoustic alarm is suppressed and the curve printout and the data sampling by a connected recorder are stopped.
The TCM3 monitor itself is a compact portable unit, but the relating calibration device, which has a volume corresponding to half the volume of the monitor (24.times.8.times.23 cm), makes the entire unit quite unwieldy.
In order to provide a measuring system which is easy to carry, a new type of calibration device has been developed with much smaller dimensions (approx. 4.times.1.times.4 cm). Said calibration device is disclosed in applicant's patent application PCT/DK89/00092, publication no. WO 90/01160. The calibration device is a mass-produced disposable device and comprises primarily a base foil into which a completely closed calibration cup is pressed, containing the fluid for calibration of a sensor.
When a sensor is to be calibrated, it is inserted into the calibration device whereby the cover of the calibration cup is ruptured, the sensor penetrates into the cup and is brought in contact with the calibration fluid.
With the newly-developed calibration device the advantage is obtained that when calibrating the electrode mentioned above need not be placed in a calibration chamber on the monitor, but calibration can be performed in close proximity to the patient. For example, on in vivo monitoring of neonates the electrode need not be removed from the incubator during calibration, as the calibration device is so small that it can easily be placed inside the incubator.
However, the calibration device does not render it possible to use the micro switch mentioned in connection with the TCM3 monitor to detect whether the electrode is present in the calibration chamber, as no electronic parts exist in the separate calibration device. To build a micro switch into the electrode is not a good solution to the problem; firstly it will make the electrode large and unwieldy and secondly the risk of the micro switch being activated by other than the calibration device is too big. In addition, the gel required in order to establish a good contact between the electrode and the skin of a patient is likely to contaminate the micro switch, causing failures and faulty detections.
However, it is very important to ensure, especially with the newly-developed calibration device by which it is possible to calibrate the electrode while located inside the incubator, that calibration is not performed while the electrode is located on a patient, but only when the electrode is located in the calibration chamber.